As is well known in the art, a solid catalyst component comprising magnesium, titanium, halogen and an internal electron donor as essential ingredients can be used in the polymerization of olefins CH2═CHR. In the polymerization process, the solid catalyst component, an alkyl aluminum compound as a cocatalyst and a siloxane compound as an external electron donor together constitute a catalyst for olefin polymerization, i.e., Ziegler-Natta (Z-N) catalyst. For example, CN1453298 discloses a Z-N catalyst consisting of: (1) a solid catalyst component comprising magnesium, titanium, halogen and a diol ester-based internal electron donor compound as essential components; (2) an alkyl aluminum compound; and (3) an organosiloxane compound containing Si—O—C group. The catalyst has good polymerization activity and stereospecificity. Nevertheless, a catalyst, which has a higher catalytic activity, better stereospecificity and better hydrogen response (that means the resulting polymer has a higher melt index at the same hydrogen amount), and is capable of resulting in a polymer with a high bulk density, is still the pursuit of researchers in this field.
In the continuous process as currently used for preparing polyolefin, two or more reactors in series are generally employed, for example, a liquid phase bulk polymerization reactor connecting with a gas phase polymerization reactor in series or two gas phase polymerization reactors connecting in series, and the like. As is well known by a person skilled in the art, examples of the former include Spheripol process and Hypol process, and examples of the latter include Innovene process and Horizone process. In the production process, the residence time and other parameters in each reactor are controlled according to the performance requirements of specific products and the process conditions, and the total residence time in individual reactors is generally controlled within 2-4 hours. This requires that the Z-N catalyst used can release its activity steadily and evenly throughout the whole process of polymerization. Or else, there will be the problem that the catalyst is too active at the early stage so that the reaction is difficult to be controlled, but not active enough at the later stage. This will directly lead to a reduction in product quality and even fail to meet the performance requirements. Thus, a stable and even release of activity of the Z-N catalyst, on the one hand, contributes to the stability and controllability of the polymerization process, and on the other hand, is favorable for adjusting various parameters in the polymerization process, thereby to produce a product with more excellent performance.
US20090253874 discloses that the catalyst can remain active for an extended period after contacting the solid catalyst component with an organic silane compound containing a functional group CH2═CH under certain conditions. However, such a compound is difficult to prepare and is expensive.
U.S. Pat. No. 5,932,510 discloses the use of a cyclic siloxane compound in a Z-N catalyst. The catalyst has a high activity, and the polymer prepared thereby has high isotacticity, high bulk density, good particle shape, and less fine dust. US6984600B2 discloses the use of polysiloxane and a cyclic siloxane for preparing a Z-N catalyst. U.S. Pat. No. 5,945,366 discloses that the use of a polysiloxane-treated Z-N catalyst in combination with a non-polysiloxane-treated Z-N catalyst can result in a polymer with high isotacticity, high bulk density and less fine dust. U.S. Pat. No. 6,200,921 B1 discloses the use of a —OH-containing organosilicon compound in a Z-N catalyst to improve the activity of the catalyst and the stereoregularity of the polymer. However, none of the above documents mentions the mandatory existence of Si—H in such polysiloxane and cyclic siloxanes, in particular the effect of such compounds for improving the stable and even release of activity of the catalyst.
The patent EP0197310 as filed in 1986 discloses the use of a Si—H containing polysiloxane in a Z-N catalyst. The preparation method of the catalyst used in this patent comprises the steps of: contacting magnesium chloride, alkoxy titanium, hydrogen-containing polysiloxane and optional alcohol to obtain a solid component; contacting the solid component with an acyl chloride compound (e.g., phthaloyl chloride), silicon tetrachloride and optional titanium tetrachloride to obtain a catalyst component; and contacting the catalyst component with an alkyl aluminum, an external electron donor (such as organosiloxane) to obtain a Z-N catalyst for olefin polymerization. When used for catalyzing polymerization under certain polymerization conditions for 2 hours, the catalyst as obtained in this patent exhibits a polymerization activity of 3.5-12.1 kg of polypropylene per g of the catalyst (calculated from the polymerization data disclosed in the examples of this patent), and the product obtained has an isotacticity of 94.4-98.8%. It is emphasized in this patent that the entire preparation process of the catalyst brings about an increase in catalytic activity of the catalyst (in relative to the level of catalyst at that time), an increase in isotacticity, and a delay of activity decay. However, nowhere does this patent disclose or imply (by comparison between examples and comparative examples) which effects would be brought about by the addition of the hydrogen-containing polysiloxane. In fact, the only two comparative examples given in this patent both point out that the stable release of activity of the catalyst is influenced (R is greater than 2 or less than 2) due to the different treatment of titanium tetrachloride, i.e. the different amount of titanium tetrachloride or the different treatment temperature. In other words, this patent is also silent about the effect of Si—H containing compound for improving the stable and even release of activity of the catalyst.
During the research and development of Z-N catalyst and the corresponding olefin polymerization, a serious problem regarding activity decay of the catalyst will be inevitably encountered, i.e., the activity of the catalyst will be significantly decreased after the reaction being carried out for 1.5-2 hours, which thereby greatly limit its use effect. After a large number of experiments and researches, the present inventors have unexpectedly found that the use of a Si—H containing organic silane compound can significantly delay the activity decay of the catalyst, so as to make the catalyst release its activity steadily and uniformly, and that the olefin polymer prepared by using the catalyst can have a significantly increased bulk density. Meanwhile, the catalyst also has high catalytic activity, good hydrogen response, and good stereospecificity.